In a sawmill in which logs are sawed into boards, planks and the like, each log is cut by advancing it lengthwise in a horizontal feed direction through a cutting machine that usually comprises one or more band saws or frame saws and may include milling cutters or the like for trimming side surfaces of the log to flatness. Ordinarily, blades or other cutters in the cutting machine are oriented vertically and define a cutting plane that extends horizontally in the feed direction. For feed through the cutting machine, each log is secured to a feed carriage that is constrained to translatory motion in the feed direction.
It is well known that the number of finished pieces that can be cut from a particular log, and the sizes of the several pieces, are dependent upon the positioning of the log on the feed carriage and its orientation in relation to the cutting plane. Heretofore it has been the usual practice to rely upon a highly skilled operator, working with manually controlled instrumentalities, to establish each log on the feed carriage in a position and orientation that would, in his judgment, result in optimum yield from the log. To this end the operator usually rotated the log to a position in which the presumbly best side was uppermost, and then oriented the log by swinging its ends sidewardly as he deemed necessary. When the log was curved along its length, it was customary to rotate it to a position in which its convex side was uppermost, so that it was as nearly as possible symmetrical to the cutting plane. At best, however, this could result in no more than a rough approximation of the optimum orientation of the log because the operator's estimate could give little or no weight to other irregularities in the geometry of the log, such as irregular taper along its length or portions having markedly flattened, oval or elliptical cross sections.
The problem of effecting a truly optimum orientation of every log is an extremely complex one because it depends not only upon the geometry of each particular log but also upon economic factors that change from time to time. The relative economic value of a given piece of finished lumber has no necessary relationship to the volume of material that it contains but depends upon its utility and upon prevailing supply and demand. A given log can often be cut according to any of two or more different cutting patterns that require different orientations and positionings of the log and produce different yields of finished pieces. In such a case, the optimum cutting pattern is of course the one that yields pieces which, together, sell for the maximum total price under prevailing market conditions.
These many factors had to be considered by the operator, individually and in their complicated relationship to one another, during the very short time that was available for orienting each of a succession of logs that had to be kept moving through the sawing machine; and his judgments had to be made without any of the measurements upon which accurate results necessarily depended. Obviously, even the most highly skilled operator could not be expected to achieve truly optimum yields with any degree of consistency.
Recent developments in the lumber industry have increased the difficulty of achieving optimum yields by manual orientation and have at the same time increased the urgency of obtaining the optimum yield from every log. Increasing demand for lumber in the face of a decreasing supply has resulted in the harvesting of younger and smaller trees and trees of more irregular shape than formerly; and the same supply and demand situation has made it imperative that existing lumber resources be conserved by obtaining optimum yield from every harvested tree. For some time, therefore, there have been intensive efforts to achieve both automatic measurement and automatic orientation of each log with the use of automatic measurement apparatus coupled with a computer that takes account of both measurement data and pertinent economic data.
U.S. Pat. No. 3,459,246 disclosed apparatus that required an operator to manually turn each log to a rotational position that he judged to be favorable for sawing, and whereby the log was then scanned by means of an arrangement of photocells that determined its smallest diameter. On the basis of that diameter measurement, band saw blades by which the log was to be cut were adjusted laterally in relation to one another to positions that would result in the presumably optimum cuts. The apparatus included no means for orienting the log laterally in the optimum lengthwise relationship to the cutting plane. Since the only dimension of the log that was taken into account was its minimum diameter, the apparatus obviously operated on the basis of data that was insufficient to ensure optimum yield and in that respect achieved little or no improvement over the results obtained with entirely manual adjustments.
The apparatus of U.S. Pat. No. 3,459,246 has a further deficiency in being so arranged that each log was delayed for a time in a zone in which it was rotated and measured, and from that zone the log was transported lengthwise to the in-feed mechanism for the sawing machine. As a result, there was a substantial interval of delay from the time when the sawing of one log was completed until the sawing of the next succeeding log would begin, and the apparatus therefore has a relatively low production capacity.
U.S. Pat. No. 3,190,323 discloses apparatus whereby, during the sawing of one log, the next succeeding log was rotated and shifted laterally to an estimated optimum position in a zone that was spaced below the in-feed mechanism. The oriented log was next translated upwardly to the in-feed mechanism, where a toothed striking member was driven into its rear end. During feed through the cutter the log was supported only by a roller under its front end and by the striking member, which also imparted forward feed motion to it. The log thus had very poor guidance and was not under positive control during cutting, so that there was no assurance of its predetermined cutting orientation being maintained.
Methods and apparatus have also been proposed whereby measurements were taken from which a calculation was made of a predetermined mathematical figure or body (parallelogram, cylinder or the like) which could be inscribed within the surface of an uncut workpiece and which was assumed to be representative for the desired scheme of cutting. The centerline of the calculated, theoretically inscribed figure or body was determined by a further calculation, and the workpiece was adjusted to bring that centerline into coincidence with the cutting plane. U.S. Pat. No. 3,970,128 discloses apparatus of this character, employed for automatic orientation of cants for their passage through an edge trimmer. Such apparatus was adequate for optimizing the yield from cants and similar essentially two-dimensional pieces, inasmuch as the assumptions upon which it operated were valid for such pieces; but with logs and similar essentially three-dimensional pieces, merely lateral adjustments are insufficient to ensure that the most profitable cuts will be made. When such apparatus is used, for example, for cutting a log having substantial lengthwise taper or a markedly non-circular cross-section, a large amount of material is wasted from one or both sides of the log, and especially from that portion of the length of the log that is nearest its root end.
Apparatus is also known for measuring the so-called sweep of a log, that is, the departure of its actual longitudinal centerline from true straightness. See for example U.S. Pat. No. 3,806,253. Again, such sweep is only one of the geometrical factors that should be taken into account in a valid calculation of the orientation in which the log must be established if it is to be cut for optimum yield.
It will be evident at this point that the problem of achieving optimum cutting orientation of a log can only be solved by obtaining a relatively large amount of measurement data by which the geometrical peculiarities of the log are fully defined. With an automatic computer there is no great problem in making the necessary calculation from the measurement data, once obtained. Furthermore, if it within the state of the art to employ automatic measurement means for obtaining the necessary data. As appears from the foregoing discussion, however, what has heretofore been lacking is complete control over all aspects of workpiece orientation, to enable data obtained from measurements to be utilized to full advantage.
It will be evident that the necessary measurements should be made with a minimum of manipulation of each log. Furthermore, since the measurements are used for the calculation of a plane that lies in the log and moves with it, the log must ultimately be established in such a position and orientation that calculated plane is parallel to the cutting plane and at a predetermined distance from it. Hence, from the beginning of the measurement process, all movement of the log must be accurately controlled in order to ensure that the cutting of the log will actually take place in exact accordance with calculations. The importance of accuracy in positioning the measured log on the feed mechanism and maintaining its orientation during cutting is apparent from the fact that an improvement in cutting accuracy of as little as 2 mm. can increase profitability by about 2%. In many cases, assuming reasonably accurate measurement, correspondingly accurate positioning of a log for sawing can enable a useful board to be produced from a side portion of it that would other wise be reduced to chips. For production efficiency, provisions for control of log orientation should not interfere with feeding of logs to the cutting machine in a steady flow, so that there is no substantial delay between successive logs.